Hatsuki is a high school student living alone with her classmate, Hatsumi, on whom she has a huge crush. Tragedy strikes on Hatsumi's 16th birthday, when she is suddenly surrounded by green light and disappears. Hatsuki manages to follow Hatsumi with the help of a being resembling a fat chick (a bird), and ends up in a place called the Great Library — a hub to many different worlds that can be visited via the books there. However, Hatsumi is nowhere to be found, so Hatsuki sets off on a journey from book to book searching for her lost love.

[Source: AniDB]